Influence of strain and cultivation procedure on the performance of simultaneous saccharification and fermentation of steam pretreated spruce

Yeast to be used in simultaneous saccharification and fermentation (SSF) of lignocelluloses materials has to be prepared in a separate cultivation step. The effects of the cultivation procedure on the performance of SSF of steam pretreated softwood were studied in the current work. The yeast used in the SSF was either directly commercially available Baker's yeast (as packaged yeast) or the same strain of yeast produced from the hydrolysate obtained in the pretreatment of the softwood material. A second strain of Saccharomyces cerevisiae TMB3000, isolated from spent sulphite liquor, was also compared with the commercial Baker's yeast. The strains were tested in SSF at substrate loads of 3, 5 and 8% dry weight of water insoluble material. Final ethanol yields were above 85% of the theoretical (based on the available hexoses) in all cases, except for the package yeast for the 8% substrate load, in which case the final yield was less than 65%. The cultivation procedure was found to have a significant impact on the performance during SSF, as well as in small-scale fermentations of hydrolysate liquor without solid material. The Baker's yeast cultivated on the hydrolysate from the steam pretreatment had in all cases a higher productivity, in particular at the highest substrate load. Cultivated Baker's yeast had a slightly higher productivity than TMB3000. The results suggest that the adaptation of the yeast to the inhibitors present in the medium is an important factor that must be considered in the design of SSF processes.